In the past, the primary field of usage of endoscopy was in diagnostics. In diagnosnics the doctor observes with the naked eye the hollow organ to be examined. The eye possesses high brightness and color dynamics, which results in the fact that the attainable imaging quality, with respect to resolution and color representation, is almost entirely dependent on the endoscope that is being used.
In surgery, however, other requirements are placed on an endoscopic system different from those in diagnostics. In the case of a surgical operation, the picture of the operating field must be available to the entire operating team, since several people carry out the surgical operation in cooperation.
As a result, observing the operating field with the naked eye through an endoscope is not adequate. For that reason, a video camera is mounted on the proximal end of the endoscope, or else a video endoscope with a distally mounted CCD sensor is used, so that the operating field can be shown on a monitor. With the use of CCD cameras, however, several problems arise as well, since a CCD sensor does not possess the dynamics, the sensitivity, and the spectral characteristics of the eye. This is especially important with regard to the representation of color. According to one possibility it is worthwhile to try to optimize the representation of color from the point of view of the color variations that occur in an endoscopic system. The optimization should be carried out as automatically as possible, so that the operating team can concentrate on the operation, and not have to concern itself with the technology.
In the case of an endoscopic examination of a hollow organ of the body, the endoscope is introduced into the body cavity or the hollow organ to be examined. Illumination light makes its way through light guide fibers in the endoscope and into the body cavity, where it illuminates the hollow organ. In order to obtain a true reproduction of the color, the illumination light should not be spectrally influenced by the hollow organ. However, depending on the nature of the hollow organ, that is not the case. A portion of the illumination light penetrates, for example, into the mucous membrane inside the hollow organ, and this membrane acts as an absorption filter. As a result, the mucous membrane acts in turn as a narrow-band spectral illumination source, since the penetrating illumination light is re-radiated in a filtered fashion from the surface of the mucous membrane, and in turn illuminates with red light the object to be observed.
This light absorption behavior differs, depending upon the hollow organ being examined. Thus, the light absorption behavior plays no role in the joint area, whereas in the stomach, where mucous membranes that are heavily supplied with blood can be found, it leads to a color shift in the red direction. For any given organ, the strength of the red shift depends upon the observation distance and the observation and illumination angle of the endoscope. The shorter the observation distance or the closer the endoscope is to the mucous membrane, the more strongly the mucous membrane is trans-illuminated, which leads to a stronger red shift.
If the angle of observation or illumination becomes greater, then the intensity of the illumination light becomes greater in the edge areas. In this case, the mucous membrane is strongly trans-illuminated there. This effect occurs in a particularly severe way in gastroscopy and coloscopy, for which the viewing and illumination angle amounts to at least 120.degree.. In such cases, tube-like hollow organs (i.e., esophagus, intestines) are being examined in which the mucous membrane is located very close to the instrument. In combination with the large viewing and illumination angle, this leads to a very large red shift in these places. However, this application-specific color shift is not taken into account by industrial video cameras.
U.S. Pat. No. 5,111,281 describes a color correction device for a video endoscope, having means for detecting the color quality of a color picture signal and means for the pixel-by-pixel execution of a dynamic color correction. Since the correction is carried out pixel-by-pixel, the known device is not able to distinguish between strong colors, red in particular, that occur in a point-like fashion, and increases in color that affect the entire picture. As a result, the known device also corrects increases in color that occur in a point-like fashion, and thus has poor color differentiation.
In U.S. Pat. No. 4,831,437 a video endoscope is described which is provided with a device for color balance. By means of this device, however, only the static color variations, such as those caused by light guide cable, endoscope, and illumination source, can be regulated in the endoscopic system. This means that dynamic color variations, such as those which can be caused by the mucous membrane during an operation, are not compensated for.
In addition, U.S. Pat. No. 4,831,437 shows a specialized device for carrying-out an automatic white balance which eliminates the color error caused by the ambient light. Spectral differences of the components being used in the endoscopic system, such as light guide, light source and endoscope, are also compensated for. For the white balance, the device has a balloon-shaped balance accessory with a white coating on the inside, through the opening of which the endoscope is inserted. It is thereby ensured that the ambient light that disturbs the balance does not mix with the illumination light. As a result, a white field of observation located in front of the endoscope is also shown in white on the monitor. If, however, a hollow organ is examined with the endoscope after the white balance has been carried out, a shift in the red direction, results from the mucous membrane.